Asphalt cement, also known as bitumen, is a sticky, black and highly viscous liquid or semi-solid form of petroleum. The primary use of asphalt cement is in road construction, where it is used as a glue or binder mixed with aggregate particles (for example, stone, sand, and gravel) to create asphalt concrete, or asphalt pavement. The asphalt cement content is typically around 5%-10% w/w of the entire mix, including the aggregate. Asphalt cement becomes hardened over time due to weathering and oxidation. Aged asphalt cement experiences a loss in ductility as it hardens, which can result in cracking and raveling of asphalt pavement structure. This effect can be explained by the change of chemical components of the aged asphalt cement. Asphalt cement is composed primarily of asphaltenes and maltenes. Asphaltenes are large, discrete solid inclusions, which are black in color and confer elastic properties. Maltenes in turn are composed of aromatic resins and saturated oils, which correlate to the viscous properties of the material. Aromatic resins are viscous polar compounds that disperse the larger asphaltene assemblies. They become fluid when heated and brittle at cold temperature. Saturated oils are clear, less viscous liquids that enable asphalt cement to flow especially at intermediate and low temperatures. Maltenes are lost during pavement service life due to three mechanisms: evaporation, exudation, and oxidation. Consequently the ratio of asphaltene to maltenes changes which leads stiffening of the aged asphalt cement.
Asphalt pavement is the most widely recycled material in the U.S., both by gross tonnage and by percentage. According to an industry survey conducted by the Federal Highway Administration and the National Asphalt Pavement Association released in 2011, more than 99% of the asphalt pavement removed each year from road surfaces during widening and resurfacing projects is reused as part of new pavements, roadbeds, shoulders and embankments. The reused asphalt pavement is also called recycled asphalt pavement/product (RAP). RAP can be used alone, but typically it is used with virgin (unused) aggregates. Since the asphalt cement in RAP is aged, it must be rejuvenated before reuse.
Asphalt rejuvenation is a process by which rheological properties of aged, hardened asphalt cement are restored to a point that the aged asphalt cement can be considered comparable to fresh asphalt cement. Asphalt rejuvenation is conducted by treating a RAP mix with recycling agents, which soften the aged asphalt cement in the mix and restore its flexibility. Recycling agents are also called softening agents, or rejuvenators. To be suitable for rejuvenating asphalt, a material must have a high flash point, be easy to disperse, have a low volatile loss during hot mixing, resist hardening, and be uniform from batch to batch. Ideally the material should have few or no asphaltenes in order to efficiently correct the asphaltene:maltene ratio.